The gripping force F of a tire can be given by the product of the frictional coefficient .mu. between the tire and the road surface and the vertical contact load W acting on the tire contact surface (F=.mu.W). In other words, the tire contact load which plays an important role in the handling of the vehicle is proportional to the magnitude of the dripping force for a given road condition.
In a known active wheel suspension system, a linear actuator which can be actively extended and retracted is typically installed between the vehicle body and each road wheel so that the distribution of tire contact load may be distributed to the four different road wheels according to a prescribed control mode. For instance, U.S. Pat. No. 4,625,993 issued Dec. 2, 1986 to Williams et al. discloses an active wheel suspension system which controls the stroke of the hydraulic actuator provided in each road wheel so that the attitude of the vehicle body may be properly controlled when the vehicle is travelling. When the vehicle is travelling straight ahead, the tires are made to follow the irregular contour of the road surface so that the gravitational center of the sprung mass may be controlled or may stay at a relatively fixed height. When the vehicle is accelerating or decelerating, the load distribution between the front axle and the rear axle is appropriately changed so that the pitching movement of the vehicle may be controlled. When the vehicle is turning a curve, the load distribution between the right and left wheels is appropriately changed so that the rolling movement of the vehicle may be controlled.
According to such a conventional active wheel suspension system, the weight of the vehicle body was simply distributed between the different road wheels, and the sum of the contact pressures of the four road wheels was essentially constant. Therefore, such an active wheel suspension system was not able to increase the overall traction or braking force of the vehicle. When a linear actuator interposed between a wheel and a vehicle body is either extended or retracted at a certain acceleration, a corresponding inertia force is produced in the sprung mass and the unsprung mass. The reaction of such an inertia force may be used to increase the contact pressure of a road wheel. However, the linear actuator used for a conventional active suspension system is so limited in its available stroke, any increase in the contact pressure that may be achieved was too short-lived to serve any useful purpose.